The present invention relates to optical filters.
Optical filters are commonly used in wavelength division multiplexed (WDM) optical communication systems. Wavelength division multiplexing increases the transmission capacity of optical communication systems (i.e., bandwidth) by allowing a plurality of optical data signals at different wavelengths to propagate simultaneously over a single optical transmission line. In order to access each of the optical data signals, one or more optical filters separate each data signal by wavelength.
Recently, there has been an explosive growth in consumer demand for bandwidth capacity. The reasons for this growth include the proliferation of Internet access and traffic, and the increasing variety of information that is transmitted over communications links, such as voice, video, e-mail, and multimedia. Because of the acute demand for high-speed, high-volume data transmission, it is desirable to increase the number of optical data signal channels transmitted over a single optical fiber without increasing crosstalk or interference between optical data signals at adjoining wavelengths.
The invention features an apparatus and method for separating an optical beam including multiple wavelength components into targeted subsets of wavelengths by causing different subsets of wavelength components either to undergo a total internal reflection (TIR) at or to transmit through internal surfaces of refractive media.
In general, in one aspect, the invention features an optical filter for spatially separating a selected subset of wavelength components from an input optical beam including multiple wavelength components. The optical filter includes: a dispersing element positioned to receive the optical beam and angularly disperse the multiple wavelength components of the optical beam as a function of wavelength; and a first internal surface positioned to intercept the angularly dispersed wavelength components and cause a first subset of the multiple wavelength components to totally internally reflect from the first internal surface and a second subset of the multiple wavelength components to transmit through the first internal surface.
Embodiments of the optical filter can include any of the following features.
The optical filter can further include a first prism containing the first internal surface.
The first prism can include at least one reflective interface positioned along a path of the angularly dispersed wavelength components between the dispersing element and the first internal surface to reflect the angularly dispersed wavelength components towards the first internal surface.
The optical filter can further include at least one additional optic positioned along a path of the angularly dispersed wavelength components between the dispersing element and the first prism to direct the angularly dispersed wavelength components towards the first internal surface. For example, the at least one additional optic can include a mirror, a lens, a refractive element, or an additional dispersive element.
Alternatively, the dispersive element can be integral with the first prism. For example the dispersive element can be a grating patterned on, or etched into, the first prism.
The dispersive element can be a grating or a chromatic prism.
The dispersive element can be an acouto-optical modulator.
The optical filter can further include a stage supporting the dispersive element and configured to adjust the orientation of the dispersive element with respect to the input optical beam.
The optical filter can further include a stage supporting the first prism and configured to adjust the orientation of the first internal surface with respect to the angularly dispersed wavelength components.
The selected subset of wavelength components can be the second subset of wavelength components (those transmitted through the first internal surface). For example, the first internal surface can be positioned with respect to the angularly dispersed wavelength to cause the selected subset of wavelengths to only include wavelengths below a wavelength upper limit. Alternatively, for example, the first internal surface can be positioned with respect to the angularly dispersed wavelength to cause the selected subset of wavelengths to only include wavelengths above a wavelength lower limit.
The optical filter can further include a second internal surface positioned to intercept one of the first and second subsets and cause a third subset of the multiple wavelength components to totally internally reflect from the second internal surface and a fourth subset of the multiple wavelength components to transmit through the second internal surface.
For example, the second internal surface can be positioned to intercept the second subset (i.e., those wavelength components transmitted through the first internal surface).
The optical filter can include a first prism containing the first internal surface and a second prism containing the second internal surface.
The selected subset of wavelength components can be the fourth subset of wavelength components. For example, the first internal surface can be positioned with respect to the angularly dispersed wavelength to cause the second subset of wavelengths to only include wavelengths below a wavelength upper limit, and the second internal surface can be positioned with respect to the second subset of wavelength components to cause the fourth subset of wavelengths to only include wavelengths above a wavelength lower limit. Alternatively, the first internal surface can be positioned with respect to the angularly dispersed wavelength to cause the second subset of wavelengths to only include wavelengths above a wavelength lower limit, and the second internal surface can be positioned with respect to the second subset of wavelength components to cause the fourth subset of wavelengths to only include wavelengths below a wavelength upper limit.
The first and second prisms can be integrated into an integral device.
Alternatively, the optical filter can further include at least one additional optic positioned along a path of the second subset of wavelength components between the first prism and the second prism to the second subset of wavelength components towards the second internal surface. For example, the at least one additional optic can include a mirror, a lens, a refractive element, or an additional dispersive element.
The second prism can include at least one reflective interface positioned along a path of the second subset of wavelength components angularly between the first internal surface and the second internal surface to reflect the second subset of wavelength components towards the second internal surface.
The optical filter can further include a stage supporting the second prism and configured to adjust the orientation of the second internal surface with respect to the second subset of wavelength components.
The selected subset can be the third subset of wavelength components.
Also, the second internal surface can be positioned to intercept the first subset, and the selected subset can be the third subset of wavelength components. Alternatively, the selected subset can be the fourth subset of wavelength components.
The optical filter can also include a single prism having both of the first and second internal surfaces.
The optical filter can also include an input fiber and a collimating lens for coupling the input optical beam to the dispersive element.
In general, in another aspect, the invention features a method for spatially separating a selected subset of wavelength components from an input optical beam including multiple wavelength components. The method includes: angularly dispersing the input optical beam as a function of wavelength to form angularly dispersed wavelength components; and directing the angularly dispersed wavelength components to contact a first internal surface along a first direction that causes a first subset of the multiple wavelength components to totally internally reflect from the first internal surface and a second subset of the multiple wavelengths to transmit through the first internal surface.
Embodiments of this aspect of the invention can include any of the features above relating to the optical filter and any of the following features.
The selected subset of wavelength components can be the second subset.
The method can further include directing one of the first and second subsets to contact a second internal surface along a second direction that causes a third subset of the multiple wavelength components to totally internally reflect from the second internal surface and a fourth subset of the multiple wavelengths to transmit through the second internal surface. For example, the second subset can be directed to the contact the second internal surface and the selected subset of wavelength components can be the fourth subset.
The input optical beam can include wavelength division multiplex (WDM) signals.
Embodiments of the invention may include many advantages.
For example, closely spaced wavelength components of a multi-wavelength optical signal can be separated from one another. This is possible because the dispersive element causes each wavelength component to propagate at different angles from one another, and subsequent internal surfaces intersecting the angular dispersed components precisely distinguish components incident above the critical angle from those incident below the critical angle. As a result, one internal surface can be oriented to precisely define an upper wavelength limit for the filter, and another internal surface can be oriented to precisely define a lower wavelength limit for the filter. Accordingly, the optical filter can be used in WDM communication systems to increase the number of optical data signals and reduce crosstalk by separating narrow bandwidth subsets of wavelength components of a multi-wavelength optical beam.
Moreover, the upper and lower limits of the wavelength filter can be tuned by adjusting the propagation angles of the angularly dispersed wavelength components and/or adjusting the orientation of one or more of the internal surfaces intercepting the components. Such adjustments permit selection of which angular dispersed wavelength components are totally internally reflected.
Other aspects, advantages, and features of the invention follow.